Light olefin materials, including ethylene and propylene, represent a large portion of the worldwide demand in the petrochemical industry. Light olefins are used in the production of numerous chemical products, via polymerization, oligomerization, alkylation and other well-known chemical reactions. These light olefins are essential building blocks for the modern petrochemical and chemical industries for the production of items such as polyethylene. Producing large quantities of light olefin material in an economical manner, therefore, is a focus in the petrochemical industry.
The production of light olefins, and in particular ethylene, can be through steam or catalytic cracking processes. The cracking processes take larger hydrocarbons, such as paraffins, and convert the larger hydrocarbons to smaller hydrocarbons products. The primary product is ethylene. However, there are numerous other chemicals produced in the process. Among the many byproducts are hydrogen, methane, acetylene, and ethane.
Historically, naphtha cracking has provided the largest source of ethylene, followed by ethane and propane pyrolysis, cracking, or dehydrogenation. Due to the large demand for ethylene and other light olefinic materials, however, the cost of these traditional feeds has steadily increased.
Energy consumption is another cost factor impacting the pyrolytic production of chemical products from various feedstocks. Over the past several decades, there have been significant improvements in the efficiency of the pyrolysis process that have reduced the costs of production.
More recent attempts to decrease light olefin production costs include utilizing alternative processes and/or feed streams. In one approach, hydrocarbon oxygenates and more specifically methanol or dimethylether (DME) are used as an alternative feedstock for producing light olefin products. Oxygenates can be produced from available materials such as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. Making methanol and other oxygenates from these types of raw materials is well established and typically includes one or more generally known processes such as the manufacture of synthesis gas using a nickel or cobalt catalyst in a steam reforming step followed by a methanol synthesis step at relatively high pressure using a copper-based catalyst.
Once oxygenates are formed, the process includes catalytically converting oxygenates, such as methanol, into the desired light olefin products in an oxygenate to olefin (OTO) process. Techniques for converting oxygenates, such as methanol to light olefins (MTO), are described in U.S. Pat. No. 4,387,263, which discloses a process that utilizes a catalytic conversion zone containing a zeolitic type catalyst. This indirect route of production is often associated with energy and cost penalties, often reducing the advantage gained by using a less expensive feed material.
Another alternative process used to produce ethylene involves using pyrolysis to convert natural gas to ethylene. U.S. Pat. No. 7,183,451 discloses heating natural gas to a temperature at which a fraction is converted to hydrogen and a hydrocarbon product such as acetylene or ethylene. The product stream is then quenched to stop further reaction and subsequently reacted in the presence of a catalyst to form liquids to be transported.
A similar process is disclosed in U.S. Pat. No. 7,208,647 in which natural gas is combusted under suitable conditions to convert the natural gas into primarily ethylene and acetylene. The acetylene in the gaseous product stream is separated from the remaining products and converted to ethylene.
More recent efforts have focused on the use of supersonic reactors for the pyrolysis of natural gas into acetylene. For example U.S. Pat. Pub. No. 2014/0058149 discloses a reactor in which a fuel is combusted and accelerated to a supersonic speed. Natural gas is injected into the reactor downstream of the supersonic combustion gas stream, and the natural gas is converted into acetylene as an intermediary product. The reaction is quenched with a liquid to stop the reaction, and the acetylene may be converted to the desired product ethylene in a hydrogenation zone.
Whether an undesired byproduct or one of the desired products, acetylene will irreversibly bond with many downstream catalysts, in particular with polymerization catalysts. Therefore, the production streams which include acetylene must be treated to remove or reduce the amount of acetylene. Additionally, in those processes that produce acetylene as an intermediary product, the majority of the acetylene must be converted to ethylene. One method of converting or reducing the amount of acetylene is selective hydrogenation.
Selective hydrogenation processes can be utilized to reduce the acetylene concentration to a sufficiently low level and can be done in either a gas phase or a liquid phase. Since selective hydrogenation is a highly exothermic reaction, the liquid phase is sometimes preferred as it can better control the temperature of the reaction. For example, U.S. Pat. No. 8,460,937 discloses a process in which acetylene is absorbed into a solvent and passed into a reactor in which a catalyst and hydrogen are present. Under proper reactive conditions, the acetylene is converted into ethylene. The solvent is described as a non-hydrocarbon polar solvent, such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), acetone, tetrahydrofuran (THF), dimethylsulfoxide (DMSO), and monomethylamine (MMA).
A byproduct of selective hydrogenation is C4+ hydrocarbons (hydrocarbons with four or more carbon atoms). The C4+ hydrocarbons are undesirable because they can accumulate on catalysts causing coke and fouling the catalyst. Additionally, the creation of the C4+ hydrocarbons needlessly consumes the acetylene and can make ethylene separation from the rest of products more complicated.
Therefore, it would be desirable to have a process which reduces the production of the C4+ hydrocarbons in a selective hydrogenation of acetylene to ethylene.
It would also be desirable for such a process that is not limited by a specific catalyst.